Zeolite is a crystalline aluminosilicate having pores having a size of about a molecule. A membrane made of zeolite has a characteristic of selectively transmitting molecules according to the size or the shape of molecules, and accordingly, such a membrane is widely used as a molecule sieve. Particularly, an application as a separation membrane for separating e.g. water and organic solvents, is attentioned.
Zeolite includes those having various structures such as A type (LTA), Y type (FAU), mordenite (MOR), beta type (BEA), ZSM-5 (MFI) and ZSM-11 (MEL). The notations in the brackets are structure codes specified by the International Zeolite Association. Further, since the size and the shape of crystal change according to production conditions, performances of separation membranes employing such zeolites are extremely various.
In general, when a membrane made of zeolite (zeolite membrane) is employed as a separation membrane, its performance is represented by permeation flux Q (kg/m2·hr) of permeation substance and separation coefficient α. Here, in a case of separating an organic material from a mixture of water and the organic material, provided that the concentrations of water and organic material before separation are A1 mass % and A2 mass %, respectively, and the concentrations of water and organic material in a liquid or a gas permeated through a membrane are B1 mass % and B2 mass %, respectively, the separation coefficient is represented by the following formula:α=(B1/B2)/(A1/A2)
As the separation coefficient α is higher, the performance of separation membrane becomes better.
For example, MOR generally has a high acid resistance, and it can be used as a separation membrane for separating an aqueous solution containing organic acid. However, as described in Non-Patent Document 1, it has a flux Q of 0.7 kg/m2·hr as measured in a separation of 50 wt % acetic acid aqueous solution, and its water permeability is low. Further, also in separation of 90 wt % isopropyl alcohol aqueous solution that is not an organic acid, MOR has low fluxes Q of 0.66 kg/m2·hr and 0.26 kg/m2·hr as described in Non-Patent Documents 1 and 2, respectively.
Meanwhile, general performances of MFI are disclosed in Non-Patent Documents 2 to 4. For example, as described in Non-Patent Document 2, in separation of 90 wt % isopropyl alcohol aqueous solution, MFI has a flux Q of 3.1 kg/m2·hr and its water permeability is higher than that of MOR. Further, as described in Non-Patent Document 3, in separation of 50 wt % acetic acid aqueous solution, MFI has a high water permeability of 3.96 kg/m2·hr. However, the separation coefficient α of MFI is lower than that of MOR, and MFI has a low resistance against acid, and accordingly, there have been many problems in long term practical use of MFI.
Meanwhile, Non-Patent Document 5 discloses a zeolite membrane having a two-layer structure constituted by a single phase of MOR and a single phase of MFI formed by two-stage hydrothermal synthesis. However, such a membrane requires the two-stage hydrothermal synthesis, and requires a high temperature heating process for removing a template. Accordingly, the process becomes complicated and its separation performance is not always good. Further, this zeolite membrane is prepared for the purpose of making a layer of single phase MFI close to a substrate function as a catalyst, and the membrane is not intended to improve separation performance. For this reason, the possibility of reducing the film thickness required to achieve improvement of separation performance is extremely low from the technical viewpoint.